Process for preparing a complex of a ligand, metal and base

ABSTRACT

This invention provides a process for the preparation of a complex of the formula L·B·M wherein (a) L is of the formula (R 1 )(R 2 )N—(CH 2 ) n —N(R 3 )(R 4 ), wherein each of (R 1 )(R 2 )N and N(R 3 )(R 4 ) is an optically active amino acid, which may be the same or different; and wherein n is an integer from 1 to 10, (b) B is a base and (c) M is a metal, the process comprising the steps of (i) providing a reaction mixture comprising L and B and having a pH of less than 7.5; (ii) contacting the reaction mixture with M to provide an first complex mixture; and (iii) contacting the first complex mixture with a further amount of B to provide a second complex mixture.

PROCESS

[0001] This invention relates to a process for the preparation of acomplex comprising a ligand, a metal and a base.

[0002] As discussed in U.S. Pat. No. 5,717,123 chelants or chelatingagents are compounds which form coordinate covalent bonds with a metalion to form chelates. Chelates are coordination compounds in which acentral metal atom is bonded to two or more other atoms in at least oneother molecule (ligand) such that at least one heterocyclic ring isformed with the metal atom as part of each ring.

[0003] Chelants are used in a variety of applications including foodprocessing, soaps, detergents, cleaning products, personal careproducts, pharmaceuticals, pulp and paper processing, water treatment,metalworking and metal plating solutions, textile processing solutions,fertilizers, animal feeds, herbicides, rubber and polymer chemistry,photofinishing, and oil field chemistry.

[0004] In the bleaching stage of photographic materials, a particularlyimportant class of bleaching agents are the aminopolycarboxylic acidbleaching agents, such as an ammonium or alkali metal salt of a ferriccomplex of ethylenediaminetetraacetic acid (EDTA) or of ethylenediaminedisuccinic acid. The production of iron ammonium salts of EDTA is taughtin U.S. Pat. No. 4,364,871 and U.S. Pat. No. 4,438,040. Ferric complexsalts of propylenediaminetetraacetic acid (PDTA) having a higherbleaching power than EDTA have also been widely used as bleachingagents.

[0005] The preparation of complexes of EDDS with metals ions and bases,such as ammonia, are known in the art. For example EP-A-0641168discloses (S,S)-ethylenediamine-N,N′-disuccinic acid iron (III) ammoniumsalt. This compound is represented by the formula:

[0006] EP-A-0641168 teaches that the above compound can be synthesisedby simple contact of (S,S) EDDS with ammonia and iron or an ironcompound. It has been recognised in the art that such prior processesfor the production of chelates comprising ligands comprising opticallyactive amino acids, such as ethylenediamine-N,N′-disuccinic acid, mayresult in undesirable precipitation of insoluble oxides and/orhydroxides. This is in contrast to the production of chelates ofproducts such as EDTA which does not result in problems of solidproduction.

[0007] U.S. Pat. No. 5,717,123 teaches that a concentrated stable ironchelate solution may be obtained by addition of a water soluble alkalimetal salt of a polyamino disuccinic acid, for example EDDS, to anaqueous ferric salt solution.

[0008] U.S. Pat. No. 5,763,634 teaches that ferric chelate of apolyamino disuccinic acid may be prepared without insolubleoxides/hydroxides precipitating out by the simultaneous addition to avessel of a ferric salt and an alkali metal salt of the polyaminodisuccinic acid.

[0009] The present invention alleviates the problems of the prior art.

[0010] In a first aspect the present invention provides a process forthe preparation of a complex of the formula

L·B·M

[0011] wherein

[0012] (a) L is of the formula (R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴), whereineach of (R¹)(R²)N and N(R³)(R⁴) is an optically active amino acid, whichmay be the same or different; and wherein n is an integer from 1 to 10,

[0013] (b) B is a base and

[0014] (c) M is a metal,

[0015] the process comprising the steps of

[0016] (i) providing a reaction mixture comprising L and B and having apH of less than 7.5;

[0017] (ii) contacting the reaction mixture with M to provide an firstcomplex mixture; and

[0018] (iii) contacting the first complex mixture with a further amountof B to provide a second complex mixture.

[0019] In the present specification by the term “base” it is meant asubstance that can accept a proton.

[0020] It will be appreciated by one skilled in the art that L offormula L·B·M is in the form of a salt and that L used in the process ofthe invention may in the form of an acid of a salt thereof.

[0021] It will be appreciated that in the process of the presentinvention B and M can be utilised in the process in ionic, elementaland/or molecular form.

[0022] We have surprisingly found that by splitting the contact of thebase B with the metal M and ligand L, such that the pH of the reactionmixture of step (i) is kept below 7.5, a process for preparing thecomplex is provided in which formation of solids is substantiallyreduced. The present process provides at least some base in the reactionmixture of step (i) and subsequently contacts further base with themixture of metal M, base B and ligand L. The split addition ensures thatthe pH is retained at a level below that which results in theundesirable solid formation.

[0023] Process

[0024] Preferably the amount of B contacted with the first complexmixture in (iii), is such that the pH of the first complex mixtureand/or second complex mixture has a value of less than 7.5. Preferablythe amount of B provided in (i), and/or the amount of B contacted withthe first complex mixture in (iii), is such that the pH of the reactionmixture, first complex mixture and/or second complex mixture has a valueof from 4 to 6. In one or more steps of the invention, or in one aspectin all steps (i), (ii) and (iii) of the claimed invention, the pH of thereaction mixture has a value of less than 7.5, preferably from 4 to 6.

[0025] During addition of M, such as ferric ions in the form of ferricnitrate, and until B such as ammonia has added in step (iii) therebyraising the pH, the reaction mixture typically reaches a pH between 1and 2. Low pH may result in decomposition of ligand L, for example ofthe decomposition of EDDS to lactam and ethylenediamine monosuccinate.Thus in a preferred aspect from the commencement of step (ii) until thecommencement of step (iii) the reaction mixture is maintained at atemperature of no greater than 30° C.

[0026] In some aspects the present process generates as by-productoxides of M, for example oxides of iron, which are insoluble under thereaction conditions. Insoluble by-products may be removed, for exampleby filtration. In a preferred aspect to minimise the amount of solidsproduced and/or to ease their filtration, the process provides astoichiometric excess of L, such as for example ethylenediaminedisuccinic acid, over M, for example ferric ion. In a preferred aspectthe ratio of M to L is 0.95 to 0.85.

[0027] In one aspect the process further comprises the step of heatingthe second complex mixture. In this preferred aspect the second complexmixture may be heated to a temperature of 60-100° C., preferably 70-90°C., more preferably approximately 80° C.

[0028] When the second complex mixture is heated, in one aspect it maybe heated for a period of 30 to 90 minutes, typically for 45 to 75minutes, or typically approximately 60 minutes.

[0029] After the constituents of the complex forming mixture have beencontacted and the mixture has been optionally heated as discussed above,the process of the present invention preferably comprises the furtherstep of adjusting the pH of second complex mixture to a value of from 6to 7.5 or 6 to 7.

[0030] The reaction medium (reaction mixture/first complexmixture/second complex mixture) of the present invention is normallywholly aqueous but the presence of other solvents such as ethanol is notexcluded.

[0031] The process of the present invention may comprise further stepsof sequentially contacting the second complex mixture with a furtheramount of M and a further amount of B. In this way further control ofthe pH and solid formation may be exercised.

[0032] Ligand L

[0033] L is of the formula (R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴) wherein each of(R¹)(R²)N and N(R³)(R⁴) is an optically active amino acid, which may bethe same or different. Preferably n is 2, 3 or 4.

[0034] The amino acids will normally be one of the 25 or so naturallyoccurring optically active amino acids listed in standard textbooks viz.alanine, valine, leucine, norleucine, phenylalanine, tyrosine, serine,cystine, threonine, methionine, di-iodotyrosine, thyroxine,dibromotyrosine, tryptophan, proline and hydroxyproline (which are all“neutral”), aspartic acid, glutamic acid and β-hydroxyglutamic acid(which are all “acidic”) and omithine, arginine, lysine and histidine(which are all “basic” and less preferred for the reasons stated below).All these acids have an α-amino group but other amino acids e.g.phenylglycine or amino acids having a β-amino group such as β-alaninecan be used. The preferred amino acids are those with two carboxylgroups and one amino group (preferably the “acidic” amino acids listedabove). Aspartic and glutamic acid are the most preferred of the three.The “basic” amino acids have more potential for unwanted side reactionsand are currently less preferred than the “neutral” amino acids. In thecase of synthetic amino acids substituted hydrocarbyl groups may bepresent. Specific optical isomers, particularly the L-form, aredesirable because they increase biodegradability and in some cases, mayalso improve the chelating effect.

[0035] Preferably L is optically active.

[0036] Preferably L is in the S isomeric form. More preferably L is inthe S,S isomeric form.

[0037] Ligand L may be selected from ethylenediamine N,N′-disuccinicacid (EDDS), diethylenetriamine-N,N″-disuccinic acid,triethylenetetraamine-N,N′″-disuccinic acid,1,6-hexamethylenediamine-N,N′- disuccinic acid,tetraethylenepentamine-N,N′″-di-succinic acid,2-hydroxypropylene-1,3-diamine-N,N′-disuccinic acid,1,2-propylene-diamine-N,N′-disuccinic acid,1,3-propylenediamine-N,N′-disuccinic acid, ciscyclo-hexanediamineN,N′-disuccinic acid, trans-cyclohexanediamine N,N′-disuccinic acid, andethylenebis(oxyethylene-nitrilo)-N,N′-disuccinic acid.

[0038] Preferably L is ethylenediamine N,N′-disuccinic acid (EDDS).

[0039] Preferably L is (S,S)-ethylenediamine N,N′-disuccinic acid or asalt thereof.

[0040] In one aspect the ligand L may be prepared in accordance with theteaching of WO 95/12570. The ligand L may be an amino acid derivativesin free acid or salt form, wherein L is prepared by a process in whichthe nitrogen atoms of two or more amino acid molecules are linked by ahydrocarbyl or substituted hydrocarbyl group, which comprises reacting,in an aqueous medium at a pH in the range 7-14 and preferably in aqueousalkali, a compound of the formula X-A-Y where X and Y are halo atomswhich may be the same or different and A is a hydrocarbyl or substitutedhydrocarbyl group in which X and Y are attached to aliphatic orcycloaliphatic carbon atoms, with an amino acid (or salt thereof),wherein the reaction is carried out in the presence of dissolved cationsof an alkaline earth metal or of a transition metal.

[0041] Ligands for use in the present invention may be prepared, forinstance, by the process disclosed U.S. Pat. No. 3,158,635. U.S. Pat.No. 3,158,635 discloses reacting maleic anhydride (or ester or salt)with a polyamine corresponding to the desired polyamino disuccinic acidunder alkaline conditions. The reaction yields a number of opticalisomers, for example, the reaction of ethylenediamine with maleicanhydride yields a mixture of three optical isomers [R,R], [S,S] and[S,R] ethylenediamine disuccinic acid (EDDS) because there are twoasymmetric carbon atoms in ethylenediamine disuccinic acid. Thesemixtures are used as mixtures or alternatively separated by means withinthe state of the art to obtain the desired isomer(s). Alternatively,[S,S] isomers are prepared by reaction of such acids as L-aspartic acidwith such compounds as 1,2-dibromoethane as described by Neal and Rose,“Stereospecific Ligands and Their Complexes of EthylenediaminedisuccinicAcid”, Inorganic Chemistry, v. 7. (1968), pp. 2405-2412.

[0042] Base B

[0043] Preferably the base of the process of the present invention B isselected from ammonia, ammonium compounds (including ammoniumhydroxide), organic amines, sodium hydroxide, potassium hydroxide andmixtures thereof.

[0044] Metal M

[0045] Preferably the metal of the process of the present invention M isiron.

[0046] In the process the metal M may be provided in elemental, ionic ormolecular form. In any of these forms the metal may be provided in theoxidation state which it is to have in the complex. Alternatively, itmay be provided in a different oxidation state to that which it willhave in the complex. In this latter aspect the process may comprise astep of modifying the oxidation state of the metal. The modification maybe an oxidation.

[0047] Preferably the metal is a transition metal. More preferably isiron.

[0048] In a preferred aspect the metal M of the formula L·B·M is atransition metal ion. In a more preferred aspect the metal M of theformula L·B·M is an iron ion, in particular Fe²⁺ and/or Fe³⁺. In ahighly preferred aspect the metal M of the formula L·B·M is Fe³⁺.

[0049] Further Preferred Aspects

[0050] In further preferred aspects L, B and M are as defined below.

[0051] L is an optically active compound is of the formula

(R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴),

[0052] wherein each of (R¹)(R²)N and N(R³)(R⁴) is an optically activeamino acid, which may be the same or different; and wherein n is aninteger from 1 to 10 preferably n is 2, 3 or 4;

[0053] B is ammonia/ammonium; and

[0054] M is a transition metal ion

[0055] L is the S,S isomeric form of a compound is of the formula

(R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴),

[0056] wherein each of (R¹)(R²)N and N(R³)(R⁴) is an optically activeamino acid, which may be the same or different; and wherein n is aninteger from 1 to 10, preferably n is 2, 3 or 4;

[0057] B is ammonia/ammonium; and

[0058] M is a transition metal ion

[0059] L is ethylenediamine N,N′-disuccinic acid (EDDS);

[0060] B is ammonialammonium; and M is a transition metal ion

[0061] L is (S,S) ethylenediamine N,N′-disuccinic acid (EDDS);

[0062] B is ammonia/ammonium; and

[0063] M is a transition metal ion

[0064] L is an optically active compound is of the formula

(R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴),

[0065] wherein each of (R¹)(R²)N and N(R³)(R⁴) is an optically activeamino acid, which may be the same or different; and wherein n is aninteger from 1 to 10, preferably n is 2, 3 or 4;

[0066] B is ammonia/ammonium; and

[0067] M is iron

[0068] L is the S,S isomeric form of a compound is of the formula

(R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴),

[0069] wherein each of (R¹)(R²)N and N(R³)(R⁴) is an optically activeamino acid, which may be the same or different; and wherein n is aninteger from 1 to 10, preferably n is 2, 3 or 4;

[0070] B is ammonia/ammonium; and

[0071] M is iron

[0072] L is ethylenediamine N,N′-disuccinic acid (EDDS);

[0073] B is ammonia/ammonium; and

[0074] M is iron

[0075] L is (S,S) ethylenediamine N,N′-disuccinic acid (EDDS);

[0076] B is ammonia/ammonium; and

[0077] M is iron

[0078] L is an optically active compound is of the formula

(R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴),

[0079] wherein each of (R¹)(R ²)N and N(R³)(R⁴) is an optically activeamino acid, which may be the same or different; and wherein n is aninteger from 1 to 10, preferably n is 2, 3 or 4;

[0080] B is ammonia/ammonium; and

[0081] M is Fe²⁺ or Fe³⁺, preferably Fe

[0082] L is the S,S isomeric form of a compound is of the formula

(R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴),

[0083] wherein each of (R¹)(R²)N and N(R³)(R⁴) is an optically activeamino acid, which may be the same or different; and wherein n is aninteger from 1 to 10, preferably n is 2, 3 or 4;

[0084] B is ammonia/ammonium; and

[0085] M is Fe²⁺ or Fe³⁺, preferably Fe³⁺

[0086] L is ethylenediamine N,N′-disuccinic acid (EDDS);

[0087] B is ammonia/ammonium; and

[0088] M is Fe²⁺ or Fe³⁺, preferably Fe³ ⁺

[0089] L is (S,S) ethylenediamine N,N′-disuccinic acid (EDDS);

[0090] B is ammonia/ammonium; and

[0091] M is Fe²⁺ or Fe³⁺, preferably Fe³⁺

[0092] The process of this invention is illustrated by the followingExamples. In each of the examples “EDDS” means(S,S)-ethylenediaminedisuccinic acid, i.e. a compound of formula:

EXAMPLES Examples 1 to 4

[0093] Examples 1 to 4 are a series of experiments showing

[0094] in comparison examples 1 and 2 the high level of solids producedby the non-sequential addition of components and/or addition to increasethe pH to above 7.5 and

[0095] in examples 3 and 4 the low amount of solids produced bysequential addition in accordance with the present invention.

[0096] Procedure

[0097] Stage 1—into a glass reaction vessel is charged ethylene diaminedisuccinic acid and water followed by aqueous ammonia solution withstirring.

[0098] Stage 2—to the so formed mixture is added a solution of ferricnitrate nonahydrate in water over 1-2 hours

[0099] Stage 3—further aqueous ammonia is added

[0100] Stage 4—the reaction mixture is heated to 80° C. for 1 hour thenfollowing cooling pH is adjusted to 6.5 to 7.5. The solution is filteredto give a clear product solution and insoluble material. EXAMPLE 1(COMPARISON) 2 (COMPARISON) 3 4 STAGE 1 Wt wet cake EDDS (g) 78 78 78 78EDDS content % 86.16 86.16 86.16 86.16 Wt EDDS in cake (g) 67.2 67.267.2 67.2 Moles EDDS 0.23 0.23 0.23 0.23 Wt water charged (g) 124 110154 137 Ammonia solution 45.4 60.5 15 30 charged (g) Concentration of25.85 25.85 25.85 25.85 ammonia solution % w/w Moles ammonia 0.69 0.920.23 0.46 charged pH resulting 9.56 10.15 5.56 6.51 STAGE 2 WtFe(N0₃)3.9H₂0 (g) 94.5 94.5 94.5 94.5 Wt water (g) 50 50 50 50 Moles Fecharged 0.23 0.23 0.23 0.23 pH resulting 2.3 4.95 0.82 1.9 STAGE 3Ammonia solution 15 0 45 30 charged (g) Concentration of 25.85 25.8525.85 ammonia solution % w/w Moles ammonia 0.23 0.69 0.23 solutioncharged pH resulting 4.90 6.60 4.4 STAGE 4 Final pH 7.01 7.01 6.78 6.8Wt final solution (g) 368.5 377.5 422.0 4.9 Wt solids (g) 1.5 1.5 0.50.5

Example 5

[0101] These examples were following the procedure below, whichexemplifies a stoichiometry of Fe to EDDS of 0.95.

[0102] Into a 1 liter reaction flask equipped with pH probe, overheadstirrer and temperature probe was charged with 360 g EDDS wet cake (at68% EDDS content equivalent to 244.8 g, 0.838 moles), 360 g water.Agitator started and ammonia solution (30% w/w) added to reach a pH of6.5. A solution of ferric nitrate (9.2% w/w ferric ion, 487 g, 0.800moles) was added via peristaltic pump over two hours.

[0103] Further ammonia solution was then added to reach pH 6.50 again.Reaction mixture was heated to 80° C. and held for 1 hour before coolingto less than 35° C. Further ammonia was added to bring pH to 7. Reactionmixture was filtered through 11 cm No54 Whatman filter paper. Example AB C D E Mole ratio Fe:EDDS 1.00 0.94 0.90 0.85 0.80 Wt product(g) 1410.41418.3 1356.1 1321.8 1283.2 Wt solids(g) 18.80 5.70 2.28 0.48 <0.1Filtration time (mins) 2.0 1.4 0.8 0.5 >10

Example 6

[0104] To a 2201 glass lined reactor is charged 65.115 kg wet cake EDDS(59.326 kg at 33.2 % LOD and 5.789 kg at 34.10% LOD) followed by 51.4 kgwater. The agitator was started and aqueous ammonia was charged to pH6.30 16.24 kg (31% solution). Temperature rose from 22.8 to 28.9° C.Ferric nitrate solution (9.19% w/w Fe) was charged over 3 hours, (81.1kg), temperature was below 29° C. Further ammonia was charged to thereactor to reach a pH of 7.2 (15.02 kg), temperature rose to 37.9° C.Reactor was discharged through a 10 micron stainless steel filter toyield 216.3 kg clear brown solution. The material had ferric content of3.23% w/w. Both lactam and ethylenediamine monosuccinate (EDMS) werebelow 0.1 g/l.

Example 7 (Comparison)

[0105] To a stirred slurry of 81.7 g EDTA in 172 g water was added 74.0g ammonia solution (26.8%) to give a solution with pH of 10.1. Asolution of ferric nitrate 166.7 g (9.5%w/w iron content) was added over10 minutes producing a dark solution. The material was stirred for 25minutes followed by heating at 80° C. for 1 hour. The mixture was cooledand pH adjusted to pH 7. The material was filtered through No 41 Whatmanpaper to give a clear solution 482 g and a small amount of solids, <0.05g. The solution had ferric content of 3.1% w/w.

[0106] This example demonstrates that in contrast to the specificcompounds of the present invention, complexes of EDTA may be synthesisedby simple addition of the complex components without the problem of highsolid formation.

Example 8 (Comparison)

[0107] To a stirred slurry of 81.8 g EDTA in 174 g water was added 36.0g of ammonia solution (26.8% w/w). To the so produced solution (pH 5.96)was added 168.1 g of ferric nitrate solution (9.5% w/w iron content).The mixture was stirred for 10 minutes then a further 36.2 g ammoniasolution was added producing a dark solution. The solution was heated at80° C. for 1 hour before being filtered through 7 cm dia No. 41 Whatmanfilter paper. Only a small amount of solids were produced, <0.05 g. Thesolution had ferric content 3.2% w/w and weighed 474 g.

[0108] All publications mentioned in the above specification are hereinincorporated by reference. Various modifications and variations of thedescribed methods and system of the invention will be apparent to thoseskilled in the art without departing from the scope and spirit of theinvention. Although the invention has been described in connection withspecific preferred embodiments, it should be understood that theinvention as claimed should not be unduly limited to such specificembodiments. Indeed, various modifications of the described modes forcarrying out the invention which are obvious to those skilled inchemistry or related fields are intended to be within the scope of thefollowing claims.

1. A process for the preparation of a complex of the formula L·B·Mwherein (a) L is of the formula (R¹)(R²)N—(CH₂)_(n)—N(R³)(R⁴), whereineach of (R¹)(R²)N and N(R³)(R⁴) is an optically active amino acid, whichmay be the same or different; and wherein n is an integer from 1 to 10,(b) B is a base and (c) M is a metal, the process comprising the stepsof (i) providing a reaction mixture comprising L and B and having a pHof less than 7.5; (ii) contacting the reaction mixture with M to providean first complex mixture; and (iii) contacting the first complex mixturewith a further amount of B to provide a second complex mixture.
 2. Aprocess according to claim 1 wherein the process comprises further stepsof sequentially contacting the second complex mixture with furtheramounts of M and further amounts of B.
 3. A process according to claim 1or 2 wherein the amount of B contacted with the first complex mixture in(iii), is such that the pH of the reaction mixture, first complexmixture and/or second complex mixture has a value of less than 7.5.
 4. Aprocess according to claim 3 wherein the amount of B provided in (i),and/or the amount of B contacted with the first complex mixture in(iii), is such the pH of the reaction mixture, first complex mixtureand/or second complex mixture is from 4 to
 6. 5. A process according toany one of the preceding claims wherein the process further comprisesthe step of heating the second complex mixture.
 6. A process accordingto claim 5 wherein the second complex mixture is heated to a temperatureof 60-100° C., preferably 70-90° C.
 7. A process according to claim 5 or6 wherein the second complex mixture is heated for a period of 30 to 90minutes.
 8. A process according to any one of the preceding claimswherein after complete addition of M, B and L, the process furthercomprises the step (iv) adjusting the pH of second complex mixture to avalue of from 6 to
 7. 9. A process according to any one of claims 1 to 8wherein the amino acid is a naturally-occurring amino acid.
 10. Aprocess according to any one of claims 1 to 9 wherein the amino acid isaspartic acid or glutamic acid.
 11. A process according to any one ofthe preceding claims wherein L is in the S isomeric form.
 12. A processaccording to any one of the preceding claims wherein L is in the S,Sisomeric form.
 13. A process according to any one of the precedingclaims wherein L is ethylenediaminedisuccinic acid or a salt thereof.14. A process according to any one of the preceding claims wherein L is(S,S)-ethylenediaminedisuccinic acid or a salt thereof.
 15. A processaccording to any one of the preceding claims wherein B is selected fromammonia, ammonium compounds (including ammonium hydroxide), organicamines, sodium hydroxide, potassium hydroxide and mixtures thereof. 16.A process according to claim 15 wherein B is ammonia.
 17. A processaccording to any one of the preceding claims wherein M is iron.
 18. Aprocess according to any one of the preceding claims further comprisingthe step of subjecting the reaction mixture, first complex mixtureand/or second complex mixture to oxidising conditions to increase theoxidation state M.
 19. A process as substantially described herein withreference to any one of Examples 3, 4, 5 or 6.